This invention relates to a method for the detection of gases in a flowing sample and, more particularly, to the use of ultrasound to induce the nucleation of bubbles and identify the bubble point in a formation fluid.
Thermodynamic properties of reservoir fluids are of great interest to reservoir engineers. One important property is the bubble point pressure (or simply xe2x80x9cthe bubble pointxe2x80x9d). Bubble point is the pressure at which gas evolves as bubbles in a fluid sample and is characterized by slowly forming thermodynamically stable bubbles. If the borehole pressure drops below the bubble point during production, gas bubbles will form in the porous reservoir rock, dramatically decreasing the relative permeability to the oil phase. Knowledge of the bubble point is also useful in determining the composition of the hydrocarbon mixture in the reservoir.
Quantitative bubble point determination is currently performed on fluid samples that have been brought to the surface. The captured fluid sample is sent to a PVT Laboratory for testing. There, the sample is placed in a cylinder, the volume of the cylinder is incrementally increased, and the pressure is monitored. Using this method, the bubble point is the pressure at which a break (knee) appears in the pressure versus volume (P-V) curve as shown in FIG. 1. This change of slope (xcex94(dP/dV)) is taken to be indicative of the bubble point.
However, PVT techniques have been found to be slow and unreliable because bubbles do not readily form at the thermodynamic bubble point of the liquid. Even when the gas phase is thermodynamically stable at a given temperature and pressure, a gas bubble may be unable to form because its surface free energy exceeds the free energy difference of the bulk phases. This phenomenon accounts for the supercooling, superheating, or supersaturation generally observed at first order phase transitions, as described by classical nucleation theory. In order to minimize the error associated with nucleation, bubble point measurements are made by changing the volume very slowly, typically over a period of an hour.
Wireline formation sampling and testing tools, such as the Schlumberger MDT, extract fluids from subsurface formations adjacent to a borehole by pumping fluids through a flowline. A primary goal of these tools is to capture samples for transport to the surface. However, one problem of these fluid sampling processes is the possibility that the fluid extracted and processed by the tool may not be representative of in-situ formation fluid. For example, if dissolved gas exists in the formation, the gas might evolve during sampling leading to erroneous measurements of fluid mobility as well as erroneous indications of free gas. Because optimal bubble point measurement techniques require identification of the first appearance of gas in the flow stream, these tools may result in unreliable bubble point measurements.
In-situ bubble point measurement using PVT (i.e., compressibility) theory is described in commonly owned U.S. Pat. No. 4,782,695 to Gotin et al., which is incorporated herein by reference in its entirety. Some problems and uncertainties of this method have been identified and partially addressed in U.S. Pat. No. 5,635,631 to Yesudas et al., which is incorporated herein by reference in its entirely.
Additional in-situ bubble point detection methods based on PVT measurements are described in U.S. Pat. No. 5,329,811 to Schultz et al. and U.S. Pat. No. 5,473,939 to Leder et al., which are incorporated herein by reference in their entireties. These methods are based on measuring pressure in the flowline as a function of flowline volume.
Another gas detection method for fluid sampling tools is discussed in U.S. Pat. No. 5,741,962 to Birchak et al. (the ""962 Patent). The ""962 patent discloses a method of determining properties of reflected and transmitted acoustic waveforms following a short initial acoustic impulse. The time delays and amplitudes of the acoustic waveforms are analyzed to find several properties of the fluid in the flowline, including the presence of bubbles.
The ""962 Patent monitors fluid acoustic impedance by measuring the amplitude of waves reflected from a fluid-solid interface necessitating the use of a delay-line crystal and a fluid-solid interface geometry that will produce good reflections. In accordance with the present invention (as discussed below), reflection measurements are not necessary and their attendant limitations are avoided. U.S. Pat. No. 5,741,962 is incorporated herein by reference in its entirety.
It has been found that for some crude oils, the slope of the pressure versus volume graph (as used for PVT techniques) does not change substantially at the bubble point. One reason for this phenomenon is that the compressibility of gas decreases as its pressure increases, thereby decreasing the compressibility contrast between gas and liquid phases. Secondly, the compressibility of the sample is the volume-weighted average of the compressibility of the gas and liquid components. Therefore, if little gas evolves at the bubble point, the average compressibility of the sample may not significantly change, even if the compressibility of the gas phase is considerably greater than that of the liquid. To deal with those situations in which the bubble point is not well marked by a compressibility change, laboratories traditionally use bubble detectors having an optical cell to detect a change in the transmission of light. As bubbles cross the optical path, the transmission of light changes. However, bubbles may form at random locations inside the measurement apparatus and may avoid detection by the optical cell. Optical methods therefore require that the bubbles be transported to the site of the sensor, such as by a stirring mechanism. However, this transport process can be inefficient. In addition, stirring mechanisms tend to be failure-prone and are not the preferred mode of transporting bubbles to the site of the bubble sensor.
Optical means have also been adapted for use in boreholes. For example, one gas detector used in fluid sampling tools is Schlumberger""s optical fluid analyzer as described in commonly owned U.S. Pat. No. 5,167,149 to Mullins et al., which is incorporated herein by reference in its entirety.
A method and apparatus for detecting marine gas seeps is disclosed in commonly owned pending U.S. Ser. No. 09/962,063 to Kleinberg et al. filed Sep. 25, 2001. Gas seeps are detected using a locally deployed probe to produce bubbles on or near the ocean floor.
Accordingly, one object of the present invention is to provide a means for making rapid, accurate measurements of bubble point.
Another object of the present invention is to provide a method of nucleating bubbles and sensing the bubble point while operating at a pressure above the bubble point pressure.
Yet another object of the present invention is to provide a gas detector method for validation purposes.
The present invention discloses a method for determining bubble point pressure and detecting gases under borehole-like conditions (i.e. temperatures and pressures typical of that experienced in borehole environments) by monitoring compressibility of the sample, the properties of the sample or the properties of an ultrasonic transducer in fluid communication with the sample. The ultrasonic transducer may be used to merely detect the presence of gas or bubbles in the sample or it may be used to nucleate bubbles in the sample prior to gas/bubble detection. The use of cavitation induced by an ultrasonic source encourages bubble formation in supersaturated samples. The samples may be captured volume samples or flowing samples. Preferably, the ultrasonic source is a piston transducer and, most preferably, it is a coaxial cylinder cell such as that disclosed in commonly owned pending U.S. Ser. No. 10/167,516 to Liang filed Jun. 12, 2002 (incorporated by reference herein in its entirety).
The properties of the ultrasonic source that are monitored include resonance frequency, voltage, voltage squared, current, current squared, phase angle between current and voltage, power dissipation, or electrical impedance, or combinations thereof. As discussed below, changes in these parameters are reliable indicators of the presence of gas or bubbles in a sample.
When bubbles have nucleated, the compressibility of the sample changes. This compressibility change is evidenced by a change in the slope of the P-V graph as shown in FIG. 1.
In addition, sample pressure, volume, temperature, harmonics and subharmonics are good indicators of the presence of gas or bubbles in the sample. For example, the sample under test will exhibit an increase in temperature at the onset of bubbles. Likewise, the appearance of or change in harmonics or subharmonics in a sample is an indicator of gas and bubble formation.
Varying the power to the ultrasonic source limits the amount of heat transmitted to the sample from the transducer. For example, providing a 0.5 to 30 W pulsed power to the ultrasonic source with 0.1 W applied between pulses continuously agitates the sample while limiting the amount of heat applied to the sample.
Accordingly, in one embodiment of the present invention, a method of fluid analysis for determining phase characteristics of a formation fluid is disclosed, comprising the steps: withdrawing a sample under borehole-like conditions; depressurizing the sample: nucleating bubble formation in the sample by activating an ultrasonic source in fluid communication with the sample; and detecting onset of bubble formation in the sample by monitoring the compressibility of the sample. The sample may be a stationary (i.e., captured volume) sample or a flowing sample. While any ultrasonic source may be used, it is preferably, a piston transducer or a coaxial cylinder cell (such as that disclosed in commonly owned pending U.S. Ser. No. 10/167,516 to Liang filed Jun. 12, 2002). The step of measuring the pressure of the sample at the onset of bubble formation may further comprise the steps of developing a first pressure-volume function of the sample prior to bubble formation; developing a second pressure-volume function of the sample after bubble formation; and extrapolating the intersection of the first and second functions, wherein the intersection represents the bubble point pressure.
In a second embodiment, a method of fluid analysis for determining phase characteristics of a formation fluid is disclosed comprising the steps: withdrawing a sample under borehole-like conditions; depressurizing the sample; nucleating bubble formation in the sample by activating an ultrasonic source in fluid communication with the sample; and detecting onset of bubble formation in the sample by monitoring the temperature of the sample.
In a third embodiment, a method of fluid analysis for determining phase characteristics of a formation fluid is disclosed comprising the steps: withdrawing a sample under borehole-like conditions; depressurizing the sample; nucleating bubble formation in the sample by activating an ultrasonic source in fluid communication with the sample; detecting onset of bubble formation in the sample by monitoring one or more ultrasonic source properties: and measuring the pressure of the sample at the onset of bubble formation.
In a fourth embodiment, a method of fluid analysis for determining the presence of gas in a formation fluid, is disclosed comprising the steps: obtaining a sample under borehole-like conditions, wherein the sample is in fluid communication with an ultrasonic source; and monitoring one or more ultrasonic source properties, wherein fluctuations in the ultrasonic source properties indicate the presence of gas in the sample.
In a fifth embodiment, a method of fluid analysis for determining phase characteristics of a formation fluid is disclosed comprising the steps: obtaining a sample under borehole-like conditions, wherein the sample is in fluid communication with an ultrasonic source; nucleating bubbles in the sample by activating the ultrasonic source; detecting the onset of bubble formation by measuring one or more ultrasonic source properties.
In a sixth embodiment, a method of fluid analysis for determining phase characteristics of a formation fluid is disclosed, comprising the steps: obtaining a sample under borehole-like conditions, wherein the sample is in fluid communication with an ultrasonic source; nucleating bubbles in the sample by activating the ultrasonic source; and detecting the onset of bubble formation by measuring one or more sample properties, wherein the sample properties include pressure, volume, acoustic radiation, transit time, amplitude, harmonics, and subharmonics and combinations thereof.
Enhanced results may be obtained by nucleating the sample twice. In the first nucleation, the sample is rapidly depressurized to obtain a rough estimate of the bubble point pressure determined by monitoring the ultrasonic source properties and/or the sample properties and/or the harmonics/subharmonics. The sample (preferably a fresh sample) is then slowly depressurized over the approximate range of the rough estimate of bubble point pressure. Again, the ultrasonic source properties or sample properties are monitored to identify the sample""s bubble point pressure.
Another embodiment discloses an apparatus useful for gas detection and bubble point measurement comprised of: means to withdraw a formation fluid sample having an ultrasonic source; and means to detect the presence of bubbles in said sample by monitoring one or more ultrasonic source properties.
Further features and applications of the present invention will become more readily apparent from the figures and detailed description that follows.